A fluctuation of a received wave, based on a fading, is a great problem for stability of communication quality in the wireless communication system. There is a diversity technique which uses two or more received waves, as a counterplan against the fading.
As classified by a configuration of branches, there are known: space diversity, an angle diversity, a polarization diversity, a frequency diversity and a time diversity.
A RAKE combining (which is a kind of the time diversity technique), is usually utilized in the wireless communication system using the spread spectrum method.
An origin of the term "RAKE" is a "rake". A meaning of the term "RAKE" is to gather signals. The diversity technique, which performs a signal combining by gathering signals which have arrived at different times because of different delays based on a multipath, is generally called "RAKE".
The diversity technique maintains a good receiving condition, by combining two or more received waves which have no correlation to each other.
In the wireless communication, there are several paths such as a direct wave and a reflected wave, where propagation conditions are different each other. Therefore, even if a condition of one received wave becomes worse, it may be possible to receive another wave via another path in a good condition. Namely, a probability, by which a good detection condition can be kept, becomes greater by combining signals through different paths as opposed to a reception of only one signal.
Some reception methods, each of which combines several diversities were proposed for applying the diversity technique to the wireless mobile communication.
In the communication system using the spread spectrum, it has been considered to combine the RAKE receiver and another diversity technique. For example, it was proposed to use the space diversity technique together with the RAKE receiver.
Referring to FIG. 4, the combination of the RAKE synthesis and the space diversity applied to the wireless communication system using sector antennas will be explained. In the example shown in FIG. 4, a base station 60 receives a signal from a mobile station 70 by using two antenna branches or an antenna 1 and an antenna 2. A receiver in the base station 60 combines received signals by using the combination of the RAKE receiver and an antenna diversity.
As shown in FIG. 4, a desired signal 1a and an interference signal 1b are included in a signal received by the antenna 1, and a desired signal 2a and an interference signal 2b are included in a signal received by the antenna 2.
A power of the interference signal, included in the signal received by the antenna branch, is approximately determined by the number of users within a sector. Therefore, when the number of users within the sector S1 is different from the number of users within the sector S2, the power of the interference signal 1a and the power of the interference signal 2a are different from each other.
Following (1), (2) are known as methods for combining received signals under the diversity detection.
(1) A signal combining in which the received signal is weighted by a value in proportion to its amplitude. An example of a circuit performing the signal combining (1) is shown in FIG. 5. PA0 (2) A signal combining in which the received signal is weighted by a value in proportion to its signal to noise ratio (SN ratio). An example of a circuit performing the signal combining (2) is shown in FIG. 6. PA0 performing a combining of received waves of RAKE fingers by using a weight value which is based on an amplitude of a received signal; and PA0 performing a combining between branches by using a weight value which is based on an inverse number of a sum of an interference power and a background-noise power in a received wave. PA0 at least two or more antenna branches; PA0 plural RAKE receivers each of which combines received waves by using a weight value which is based on an amplitude of the received wave, wherein the number of the RAKE receivers is the same number of the antenna branches; PA0 an apparatus which measures a sum of an interference and a background-noise power in an output signal from each RAKE receiver; and PA0 a branch combiner which multiplies the output signal from each RAKE receiver by an inverse number of said measured sum, and which sums the value obtained by said multiplication.
In FIGS. 5 and 6, the number of fingers of each RAKE receiver is 2. Each RAKE receiver is connected with each antenna branch. The number of paths to be received is 2.
FIG. 5 shows a signal combining apparatus in which the received signal is weighted by a value in proportion to its amplitude. The apparatus comprises the RAKE receiver 10, the RAKE receiver 20 and the adding circuit 53. The received signal from the antenna 1 is input to the RAKE receiver 10. The received signal from the antenna 2 is input to the RAKE receiver 20. Each of the RAKE receivers 10, 20 comprises two correlation apparatuses (correlators) 11, 14, two amplitude measuring apparatuses 12, 15, and two integrating circuits 13, 16.
In the signal combining apparatus shown in FIG. 5, the signals from the antenna branches 1, 2 are converted to base-band signals the, and quantized by A/D converter, then converted to digital signals. These digital signals are input to the RAKE receiver 10, 20 respectively. In each of the RAKE receivers 10, 20, a despreading of the input signal is performed, by each of the correlators 11, 14, by using a signal sequence which is used for the spread spectrum. Each amplitude of output signals from the correlators 11, 14, is measured by each of the amplitude measuring apparatuses 12, 15. Each output signal from the correlators 11, 14 is weighted, in each of the integrating circuits 13, 16, by using a value in proportion to the measured amplitude. The weighted signals are applied to an adding circuit 53. The circuit 53 obtains a sum of the signals from the integrating circuits 13, 16 respectively in the RAKE receivers 10, 20. Then, a combining between fingers and a combining between branches are performed.
FIG. 6 shows a signal combining apparatus in which the received signal is weighted by a value in proportion to its SN ratio. The apparatus comprises the RAKE receiver 10, the RAKE receiver 20 and the adding circuit 53. The received signal from the antenna 1 is input to the RAKE receiver 10, and the received signal from the antenna 2 is input to the RAKE receiver 20. Each of the RAKE receivers 10, 20 comprises two correlators 11, 14, two signal to noise ratio (SN ratio) measuring apparatuses 18, 19, and two integrating circuits 13, 16. In comparing FIG. 6 with FIG. 5, two SN ratio measuring apparatuses 18, 19 are used in place of two amplitude measuring apparatuses 12, 15.
In the signal combining apparatus shown in FIG. 6, the signals from the antenna branches 1, 2 are converted to base-band signals, and quantized by A/D conversion, then converted to digital signals. These digital signals are input to the RAKE receiver 10, 20 respectively. In each of the RAKE receivers 10 and 20, a despreading of the input signal is performed by each of the correlators 11, 14 by using a signal sequence which is used for the spread spectrum. Each SN ratio of output signals from the correlators 11, 14 is measured by each of the SN ratio measuring apparatuses 12, 15. Each output signal from the correlators 11, 14 is weighted in each of the circuits 13, 16 by using a value in proportion to the measured SN ratio. The weighted signals are input to the adding circuit 53. The adding circuit 53 obtains a sum of the signals from the circuits 13, 16 respectively in the RAKE receivers 10, 20. Then, the combining between fingers and the combining between branches are performed.
In the combining method (1) using the weight in proportion to the received signal amplitude, a required circuit scale of the signal combining apparatus is small, because the signal combining can be performed only by measuring an instantaneous value of the signal power in the amplitude measuring apparatuses 12, 15. However, because the power of the interference signal included within the received signal from the antenna 1 is different from the power of the interference signal included within the received signal from the antenna 2 as mentioned above, if the combining method is applied to the communication system shown in FIG. 4, it is impossible to obtain the maximum SN ratio of the combined signal in the signal combining apparatus shown in FIG. 5. Then, a characteristic of the signal combining becomes worse.
On the other hand, it is possible for SN ratio of the combined signal to become the maximum in the signal combining apparatus shown in FIG. 6, because the signal combining is performed by using the weight in proportion to SN ratio of the received signal.
However, for measuring SN ratio, the apparatuses 18, 19 are required to measure a variance or dispersion of a noise in the received signal. Further, for measuring the variance of the noise, a long-time measurement and an average process of the measured values are required.
Therefore, in the signal combining apparatus shown in FIG. 6, a hardware scale such as a memory amount in the SN ratio measuring apparatuses 18, 19 becomes larger than the amplitude measuring apparatuses 12, 15.